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Ultimate GPS v3 - Lagging speed reporting by as much as 27 s
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Ultimate GPS v3 - Lagging speed reporting by as much as 27 s

by Keep_Looking_Up on Wed Aug 26, 2020 6:01 pm

I've been doing some testing with a recently purchased Ultimate GPS Breakout v3 PA6H board, and I'm getting very strange results for speed. It seems to lag and/or be some kind of running average instead of actual speed. I've compared the GPRMC output with a Garmin GPS18 LVC while driving around the block and the results are in the attached graph (x axis is in seconds). This data comes from the actual NMEA sentences coming directly from the GPS units, not from the Adafruit library.

adavsgarmin.png
adavsgarmin.png (58.71 KiB) Viewed 24 times


The speed output of the Adafruit GPS board lags a lot, and when I stop, it takes a while to actually get back down to 0. I started off in my car and accelerated to about 30 MPH, and the Garmin keeps pace very closely with the car speedometer (though in knots, of course) and when I stop at a stop sign, the Garmin drops rapidly to 0. When I first started moving, the adafruit board stayed close to zero for about 10 seconds before it even started to rise up.

Within a second or two of starting to move, the Garmin board was showing increasing speed. The Adafruit board was hovering around 0.5 as it had been doing while stopped.
15 seconds after I started moving, the car was about 30 MPH, the Garmin was reporting 25 knots, and the Adafruit board was reporting only 2 knots.
Gradually the Adafruit board finally caught up to the Garmin 27 seconds after I started moving.
Then when I slowed down to a stop, the Garmin followed and went to 0, but the Adafruit board took 4 seconds to get below 2 knots, and then hovered around 1.4 knots, never really making it to close to 0.

I see this behavior at both 1HZ and 5HZ output rates. If you simply output the Adafruit speed to a display like a speedometer and just drive around, it is very obvious something is wrong. I've never seen anything like this in the NMEA output of several other GPS boards.

I found another forum post that mentions what sounds like the same problem from 2016, but no solution. viewtopic.php?f=31&t=107220&p=537363&hilit=GPS+SPEED#p537363

I've read the docs that are available on the GPS chip used in the board looking for some kind of setting that might affect the speed processing, but have found nothing.

Any ideas?
Thanks

Keep_Looking_Up
 
Posts: 4
Joined: Sat Aug 22, 2020 8:17 pm

Re: Ultimate GPS v3 - Lagging speed reporting by as much as

by jferguson on Tue Sep 15, 2020 9:18 am

This may not be what the problem is, but ten years ago I bought a gps module for an arduino-based anchoring drag alarm. The ideas was that this device which looked at wind, heading, and depth, would alarm if our boat dragged at anchor. I had a lot of trouble with the gps noty detecting slow moevement. It turned out that it was firmware set to sleep if movement was less than one meter in 3 seconds (IIRC). Of course nothing in the notes, so emailed the manufacturer in Shanghai and after a month, got a nice note saying, that the chip was intended for use in cars and the sleep was to mask jitter. They'd found that car manufacurers wanted the position to stick when the car was not moving. There was a couple of bytes to send to the chip on startup to get it to stay awake. I could do it when activating the device, but shanghai engineer suggested I send the code on every read. It worked.

It worked fine for me, and the alarm also worked. my code did assume that a drag would be downwind or if not would involve a depth change. Don't ask me what I did, I can't remember now.

jferguson
 
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Joined: Mon Aug 20, 2012 2:29 pm

Re: Ultimate GPS v3 - Lagging speed reporting by as much as

by jferguson on Tue Sep 15, 2020 9:22 am

is there no way to edit a messed up post?

jferguson
 
Posts: 21
Joined: Mon Aug 20, 2012 2:29 pm

Re: Ultimate GPS v3 - Lagging speed reporting by as much as

by Keep_Looking_Up on Tue Sep 15, 2020 5:25 pm

Thanks for the reply jferguson. I'm familiar with the setting you're talking about. The low level random jitter in gps positions will normally cause the position, bearing and speed to vary randomly by small amounts when you're sitting still. There's a setting to force nav updates to stop when the speed is below a certain level. This effect I'm seeing isn't that.

No, this appears to be some serious problem with the MTK3339 speed processing. After further testing showed the speed problem was something I couldn't work around in my project (and I was worried the bearing and position accuracy might be similarly affected), and with no solution, I abandoned this device and switched to another type GPS module which outputs speed correctly.

Keep_Looking_Up
 
Posts: 4
Joined: Sat Aug 22, 2020 8:17 pm

Please be positive and constructive with your questions and comments.